For an encapsulant for an LED, it has been desired a material excellent in heat resistance, light resistance, workability, adhesiveness, gas barrier property and curing characteristics, and a thermoplastic resin such as an epoxy resin, a poly(meth)acrylate, and a polycarbonate, etc., has heretofore been often used. Due to a recent trend toward higher output of LED light emitting apparatus, however, it has come to be found that use of these thermoplastic resins causes a problem of heat resistance or discoloration resistance under a condition of a high temperature over a prolonged period of time.
In addition, in recent years, when an optical device is to be soldered to a substrate, a lead-free solder has been used in many cases. This lead-free solder has higher melting temperature as compared with that of the conventional solder, and it is required to carry out soldering generally raising a temperature of 260° C. or higher. When soldering is carried out at such a temperature, in the conventional encapsulant comprising the thermoplastic resin, it has also been found that inconveniences occur, e.g., deformation occurs or the encapsulant becomes yellowing due to high temperature, etc.
As described above, encapsulants are required to have more excellent heat resistance compared to previous ones due to a trend toward higher output of LED light emitting apparatus and use of lead-free solders. Until now, a resin composition for optical purpose in which nano silica is filled in a thermoplastic resin, etc., has been proposed for the purpose of improvement in heat resistance (Patent Literatures 1 and 2), but in the thermoplastic resin, there is a limit in heat resistance, and sufficient heat resistance cannot be obtained.
A silicone resin which is a thermosetting resin is excellent in heat resistance, light resistance, and light transmissivity, so that it has been investigated to use the same as an encapsulant for an LED (Patent Literatures 3 to 5). However, the silicone resin is weaker in resin strength as compared with that of an epoxy resin, etc., and has large gas permeability (i.e., gas barrier property is low), so that there is a defect that luminance is lowered by sulfurization of an electrode, etc.
Also, for example, when a silicone resin containing a silicate-based phosphor is used for an encapsulant for an LED, water vapor is migrated into the encapsulant of the silicone resin having low gas barrier property, so that there is a problem that water is reacted at the surface of the phosphor to decompose the phosphor, whereby fluorescent characteristics are markedly lowered. Thus, when the conventional silicone resin is used for encapsulant for an LED, there are problems that long-term reliability of the LED under highly humid is lowered, in addition to the problem that luminance is lowered due to sulfurization of an electrode, etc., so that a demand to improve gas barrier property of the silicone resin has been increased.
As a measure thereof, it has been investigated to make refractive index high and to improve gas barrier property by introducing an aromatic substituent such as a phenyl group, etc. However, if the aromatic substituent is introduced, change in viscoelasticity at heating becomes large, so that there are problems that crack resistance is lowered as compared with a methylsilicone resin, or adhesiveness to the LED package is worsened.
For the improvement in adhesiveness, it has been investigated to blend an epoxy group-containing organopolysiloxane or a nitrile compound (Patent Literatures 6 and 7), but heat resistance is lowered so that an amount to be blended is limited, whereby it cannot be said that sufficient adhesiveness can be obtained. Therefore, development of an encapsulant excellent in heat resistance, having good workability, excellent in mechanical characteristics, heat resistance and adhesiveness of the cured product, and further excellent in gas barrier property has been required.